The present invention relates to an image forming output apparatus such as a digital color copying apparatus, a digital color printer, and the like to which an electrophotographic process is applied.
In a copying apparatus or printer to which an electrophotographic process is applied, an image is formed in the following manner: a rotating cylindrical photoreceptor or a belt-shaped photoreceptor is rotated and electrostatic latent images are formed thereon successively; black toner and other color toners, in the case of color image formation, are adhered to the electrostatic latent images formed as described above for development; and they are transferred onto a recording sheet, thus, the image is obtained. In this specification, the photoreceptor drum in the image output apparatus and a driving roller for the belt-shaped photoreceptor are referred to as a rotational body. When the rotational speed of the photoreceptor drum is varied for some reasons, jittering or an uneven image is caused in the outputted image. These phenomena have remarkably appeared especially in the digital system electrophotographic technology employing scanning by means of a semiconductor laser for writing images on a photoreceptor. Fluctuation in rotational speed of the photoreceptor has caused speed fluctuation of a writing system in the subsidiary scanning direction to create a slight variation in the distance between writing lines, contributing to the remarkable deteriorations of the image quality.
Conventionally, when designing the driving system for use in a copying apparatus or a printer, the main consideration is that the objects driven by the driving device are appropriately located in the allowable space, while satisfying the values of the line speed or number of revolutions introduced from the product specification. That is, the following are main concerns: the method by which the driving power is transmitted from a driving power source to a driven object; and mechanical elements for power transmission. Accordingly, when jittering and rotational fluctuation are caused in the product, the cause is investigated, and one or more of the following countermeasures are considered: a bearing of a drive shaft of the photoreceptor is replaced with one made of sintered metal; a flywheel is connected with the drive shaft of the photoreceptor; a brake, in which a spring is combined with a friction material, is provided on the rotary shaft of the photoreceptor drum; the accuracy of a gear is enhanced; or a helical gear with various kinds of torsion angles is provided.
However, in the development of a digital type image output apparatus, strict reproducibility of a one dot line written by a laser beam is required with an improvement of the apparatus performance, and accuracy required on the driving system has rapidly become strict. The accuracy required is a level at which the uniformity of laser writing in the subsidiary scanning direction is assured in relation to the visible sensitivity of the visual system. In order to accomplish this accuracy, it is mostly necessary to make the photoreceptor driving system highly accurate. The main factor of the rotational fluctuation of the driving system is the following: the rotational fluctuation per one rotation of the rotating shaft of a motor is large, and absolute values of fluctuation components per one rotation of a gear and per one tooth of a gear are large; and fluctuation components and their higher harmonic wave components cause a resonance phenomenon in relation to the proper oscillation frequency of the driving system.
FIG. 14 shows the power spectrum of speed fluctuation of conventional apparatuses. In FIG. 14, fluctuation components of a gear according to the line speed proper to the apparatus are 176 Hz in the case of a gear directly coupled to the motor, 64 Hz in the case of a second shaft, and 25 Hz in the case of a gear directly coupled to a drum, and in this case, a higher harmonic wave component of 50 Hz is shown. Further, a component of a rotation of the gear directly coupled to the motor is 22 Hz, and its higher harmonic wave component of 44 Hz is shown in the drawing.
FIG. 15 shows an example in which a transfer function for obtaining numerically the proper oscillation frequency of the driving system was measured. In this case, the measurement was conducted in the following way: an output of an impact excitation hammer, and an output of a piezoelectric type pickup sensor, provided to one end of a photoreceptor drum in order to measure the fluctuation of the acceleration in the rotation direction, were connected with a dual channel type FFT analyzer; and a Fourier spectrum ratio was obtained. From FIG. 15, the following can be found: a peak of the proper oscillation frequency is near 45 Hz; and high level areas of the transfer function are spread to cover the range of 30 to 60 Hz.
FIG. 16 shows superimposition of the fluctuation component spectrum and the transfer function. In the driving system, it can be found from the drawing that a peak of the transfer function and the position of a frequency area, to which the fluctuation component and its second harmonics belong, are overlapped. That is, it is found that the driving system amplifies the fluctuation components (resonance is caused).
Actually, when measurement data from three apparatuses each having the present driving systems were investigated, the fluctuation of rotation of the photoreceptor showed values of 5 to 8%.
In order to solve the foregoing problems, in the present invention, decrease of the speed fluctuation of a motor and a gear was taken as a an assumption. Further, in addition to that, a driving device for a rotational body was constituted as follows, with the transmission of the fluctuation component in the driving power transmission system, and the concept of the transfer function, resonance and proper oscillation frequency as well as a method to reduce the transmitted fluctuation, all taken into consideration.
At first, in order to prevent the resonance of the driving system for the rotational body, matching of the proper oscillation frequency of the driving system with the frequency of the fluctuation component transmitted to the driving system was avoided. Generally, the proper oscillation frequency .omega. is expressed by the following equation. ##EQU1##
In the equation, K indicates the torsional rigidity of the driving system, and I indicates its moment of inertia. In order to prevent the resonance, the value of .omega. can be changed by changing the value of K or I. In order to prevent the resonance, the value of .omega. may either be larger or smaller with respect to the fluctuation component of the driving system. In order to make the value of .omega. larger, the value of K is increased or the value of I is decreased. On the other hand, in order to make the value of .omega. smaller, the value of I is increased or the value of K is decreased.
With respect to the driving system having data shown in FIG. 14, FIG. 17 and FIG. 18 show the power spectrum of the rotational fluctuation and the actually measured value of the transfer function of the driving system in the case where the proper oscillation frequency is changed to a larger value and a smaller value by changing the structure of the driving system, wherein data are overlapped in the same manner as that in FIG. 16. FIG. 19 shows the comparison of peak values of the transfer function of the foregoing three driving systems. In FIG. 17, when the structure of the driving system is changed for decreasing the rigidity, the value of K, which is the torsional rigidity of the driving system, is decreased. When the data shown in FIGS. 17, 18 and 19 are compared with each other, the transfer function is decreased together with a change of the number of the proper oscillation in the case where the structure is changed in order to reduce the torsional rigidity K of the driving system so that the proper oscillation frequency is changed to a lower frequency side, wherein the proper oscillation frequency is changed in order to prevent the resonance. It is considered that the foregoing effect can be obtained in the following manner: when the structure is changed in order to change the proper oscillation frequency, the attenuation coefficient is increased due to the change to the flexible structure; and the structure is changed so that the rotational fluctuation is absorbed in the driving system itself. From above effects, the following is found: when the structure is changed in order to reduce the torsional rigidity K of the driving system in the case where the proper oscillation frequency is changed in order to prevent the resonance, the value of transfer gain of the fluctuation of the rotational speed is changed; and this is advantageous and effective in that the fluctuation of the speed of the driving system is decreased. On the other hand, when the structure is changed in order to reduce the rigidity K, the fluctuation of the speed is hardly absorbed in the case where force is instantaneously applied when a cleaning blade or the like, for example, is brought into contact with a member of the system, so that the above influence remains on the image for a long period of time.
In an image output apparatuses, especially in digital type image output apparatuses in which an image writing operation onto the photoreceptor is conducted by a laser beam, an image with high quality can not be obtained when the rotational body is not rotated constantly at a predetermined speed. When the rotational speed is changed, distance between scanning lines of the laser beam is different at each position, so that ununiform image density is generated. Further, in the case of a color image, colors, generally yellow, magenta, cyan, and black toners are superimposed, and therefore the color toner balance is lost and the color reproducibility is lowered.
However, when the impulsive force is applied to the rotational body of an image output apparatus, for example, a cleaning blade, which is used for wiping-off toners adhering to the photoreceptor surface, is sometimes brought into contact with the photoreceptor surface, upon which the periodical fluctuation of the rotational speed of the rotational body is generated. Further, the speed fluctuation remains for a long period of time, a long period of time is necessary for eliminating the speed fluctuation, and the image is largely disturbed during the time, which is a problem.